In general, during electric discharge machining, an electrical discharge induced by a machining pulse between a workpiece and an electrode, results in the removal of a small crater of material from the workpiece. Electric discharge machining is performed by the application of a plurality of electrical discharges in a highly repetitive manner. The spacing between the tool electrode and the workpiece, called the machining gap, is typically on the order of a few microns or tens of microns in size and is typically filled with a dielectric machining liquid, such as water or kerosene. The machining gap may be maintained at a generally constant size by comparing mean gap voltage detected at the gap during machining to a reference servo-feed voltage, and using the comparison result to control machining conditions.
During machining, a machining pulse having an ON state and OFF state under predetermined machining conditions is generated, which in turn produces a current pulse having a nearly constant peak current value and constant pulse width. When preparing to machine, the operator sets machining conditions such as the on-time and off-time of the machining pulses applied across the gap, the peak current value, detection levels for detecting unstable machining, as well as other parameters in accordance with requirements such as the machining area of the workpiece, the machining depth, the dimensional accuracy required, and the surface roughness desired. As machining progresses, the operator may regulate the reference servo-feed voltage. Further, based on his experience or by referring to recommended data table, the operator may regulate a jump cycle and a jump stroke of tool electrode jump action, whereby the electrode is moved up and away from the workpiece and then immediately returned down to a position close to the workpiece, to thereby create a pumping action in order to remove particles produced during machining from the gap. It is known that such particles may result in a decreased material removal rate if left in the gap.
Conditions such as reference servo-feed voltage, the jump cycle and the jump stroke must be regulated so that the conflicting requirements of a high material removal rate and stable discharge machining are effectively balanced. For example, if the jump cycle is shortened and the jump stroke increased, while the stability of discharge will be improved, the material removal rate will be decreased. Therefore, fairly good skill is required on the part of the operator to properly regulate conditions during machining. For example, a skilled operator may observe such conditions as discharge frequency, the waiting time for discharges, i.e., the time during which no discharge occurs even though voltage has been applied across the machining gap, the voltage during the waiting time for discharge and the occurrence of short-circuiting, all by referring to waveforms of the gap voltage using an oscilloscope.
U.S. Pat. No. 5,117,083 issued to Kawamura discloses a jump control system for controlling jump action conditions using a so-called "fuzzy" inference control instead of the intuition and experience of the skilled operator. In this disclosure, in order to control a jump cycle and a jump stroke, the stability of the discharges and the rate of change are inferred based on given fuzzy rules according to detected gap conditions such as the waiting time for discharge, the gap voltage during the waiting time, and the mean voltage.
However, in order to decide whether gap conditions are good or bad, values representative of the gap condition are compared to respective given reference values, which in turn are electrically detected by detecting means. Since reference values must be changed according to changes in the machining conditions, many reference values are set in advance.